The invention relates to joint transform correlators and particularly to a joint transform correlator employing an interferometer, and a data formatting system using frequency multiplexing techniques.
Optical joint transform correlators used for real time applications employ two spatially separated images written onto the input transducer. Systems not used for real time use film images separated spatially at the input, or an image at the input and a holographic matched filter at the transform plane.
A joint transform correlator using two spatially separated input images has real time applications since the images can be written onto a spatial light modulator (SLM). The input images are then illuminated by a collimated coherent light source. The spatially modulated light is then passed through a lens to produce the joint transform plane. Since the input images are spatially separated, and the light from each image passes through the transform plane with a given angular relationship, interference fringes are produced. This process optically introduces a carrier to the cross correlation products. To complete the correlation process it is necessary to multiply all of the cross terms from each of the input images. The multiplication is accomplished by using an optical detection process, such as recording the joint transform on film (a non-linear detection media) or, for real time operation, the joint transform can be written onto an optical-to-optical transducer such as a liquid crystal light valve. The output of the second transducer is then transformed optically to produce the correlation plane. Since a carrier is introduced by spatially separating the input images, the cross correlation output is spatially separated from auto correlation and other undesired outputs produced by the optical system.
Under real time conditions, such a prior art correlator system requires a complete input system. Generally the spatial light modulators used in the system have a single input port. Thus, in order to receive the two spatially separated input images, the system must employ a digital data formatting system that further includes a significant high speed memory system with proper digital support. If the input images contain broadband signal information in analog format, high speed digitation is also required. Thus, such prior art correlator systems require relatively complex digital input circuits, and do not make efficient use of the area space bandwidth product of the spatial light modulator, i.e., of the input transducer.
A second prior art correlation system employs two separate light transducers which require extensive optical and electrical matching that generally is cost prohibitive in producing a high time bandwidth correlation system.
The above prior art joint transform correlator, using spatially separated input images, has a time bandwidth product equal to one third of the time bandwidth product of the input transducer, since the input image covers one third of the transducer's area. On the other hand, the frequency multiplexed joint transform system of description herein has one half the time bandwidth product of the input transducer, since the entire transducer area is used, at one half the bandwidth.
Thus, the invention correlator system extends the time bandwidth product of the joint transform correlator system, while also eliminating the need to data format with a complex digital system. The frequency multiplexed joint transform correlator system allows each channel to use the entire scanned area of the input transducer while each channel occupies one half of the total allowable bandwidth. This system thus allows the input transducer to be used more efficiently.
A typical prior art cross correlator system employing two spatially separated input images is shown in the article by D. Casasent, Proceeding of the IEEE, Vol. 67, No. 5, May 1979, pages 813-825. A prior art system employing two input transducers and a dual-axis configuration is depicted in the article by T. C. Lee et al, Optics Letters, Vol. 4, No. 4, Apr. 1979, pages 121-123. Still another prior art correlator system employs a form of signal multiplexing at the input thereto, but which further employs electronic means (i.e., a modified spectrum analyzer) not optical means, to process the joint transform correlator output. Such a system is depicted in the article by D. Casasent, et al, Applied Optics, Vol. 17, No. 21, Nov. 1, 1978, pages 3418-3423.